The validation of data that is incorporated into an image is always necessary in circumstances in which the image is used, together with the data, for evidentiary purposes, as takes place for example in the automatic monitoring of traffic, where infringements of traffic regulations are contemporaneously recorded in a tangible form. This may occur, for example, where an offending vehicle is photographed and the corresponding data such as speed, date and time are incorporated into the image on the film, in the form of alphanumeric symbols. In this case, the conversion of electrical data signals into the symbols to be transferred optically onto the film takes place by means of known display elements, such as for example light-emitting diodes.
To demonstrate that the process of adding the data to the image is correct and the symbols which are visible on the image actually represent the data intended to be incorporated into the image, it is known inter alia to monitor the electrical properties of the display elements. This preferably takes place by detection or measurement of the current through the light-emitting diode segments during the process of adding the data. Since, however, there is no guarantee that a light-emitting diode exhibiting the expected electrical properties will also behave correctly from the optical point of view, in that it actually also illuminates for example when current flows through it, this method is not sufficient for evidentiary purposes.
Another known method consists in providing, at the start and possibly also at the end of each film, test exposures with predetermined data or patterns that are added to the images, and checking these subsequently. Even this method is not unequivocally reliable evidence, since it does not guarantee that the incorporation of the data has also correctly functioned in the case of any desired image before or after the test exposure.